The bandwidth shortage increasingly experienced by mobile carriers has motivated the exploration of the underutilized Millimeter Wave (mmWave) frequency spectrum between 3G and 300G Hz for the next generation broadband cellular communication networks. The available spectrum of mmWave band is two hundred times greater than the conventional cellular system. The mmWave wireless network uses directional communications with narrow beams and can support multi-gigabit data rate. The underutilized bandwidth of the mmWave spectrum has wavelengths ranging from 1 mm to 100 mm. The very small wavelengths of the mmWave spectrum enable large number of miniaturized antennas to be placed in a small area. Such miniaturized antenna system can produce high beamforming gains through electrically steerable arrays generating directional transmissions.
Robust signaling and data forwarding in beamformed mmWave systems demands new design. Known issues including unreliable TX/RX paths, random radio link failure (RLF) and service interruption, and consequently the performance degradation particularly in mobility. For example, connection failure and slow handover (HO) may occur due to RLF. A radio link failure that occurs in the source network is likely to be caused by too late handover. Likewise, a radio link failure that occurs in the target network is likely to be cause by too early handover. HO to the wrong target network and unnecessary HO to another RAT may also occur.
The existing LTE mobility is complex but based on omni-directional antenna without beamforming. In general, LTE smallcell mobility can be used as the baseline for a standalone mmWave smallcell. However, the heavy reliance on directional transmissions and the vulnerability of the propagation environment present particular challenges arise from channel characteristics and beamforming in mmWave small cells. For example, directional antenna and beamforming makes signaling path and data path even less reliable than omni-directional systems because more intermittent links and limited wireless coverage need to be compensated. Multiple levels of beams, multiple beams per level, and multiple TDM beamformed control beams need to be tacked, switched, and aligned with, resulting in complex time-critical decision to tradeoff among robustness, spatial coverage, speed, and link budget. Furthermore, different spatial paths offered by different levels of dedicated and control beams result in different channel coherent time and fading dynamics. Multiple choices of spatial beams thus offer more spatial diversity to be explored in mmWave small cells.
A solution of achieving spatial diversity for enhancing the reliability and performance of data and control path in mmWave smallcell system is sought.